Antigen-specific T cell responses in mucosal tissues: Mice were immunized with live cells of an enteric pathogen, and bacterial translocation into tissues was followed over time. T cells from various lymphoid organs were then cultured in vitro with soluble bacterial sonicate antigen [Ag] and Ag-specific proliferation and lymphokine-secretion were assayed. Results indicate that CD4+ cells are responsible for all three parameters studied, and that the appearance of IFN-g secreting T cells in tissues closely follows bacterial translocation from the gut. Thus, they appear in mucosal lymphoid organs [PP and MLN] before they appear in the spleen. The response also dies out faster in PP and MLN than in the spleen [4-6 wks vs 9 wks]. However, when mice are challenged with bacteria at 17 weeks, and T cells stimulated in vitro a week later, IFN-g secreting T cells are found only in the PP and MLN, and not in the spleen, indicating that while IFN-g secreting T cells might leave these sites early, quiescent 'memory' T cells might persist there. T cells in the lamina propria of the gut were then tested, and it was observed that IFN-g secreting T cells were present both at 4 and 9 weeks after immunization. However, they responded only to polyclonal stimulation, not to specific antigen. This indicates that some sort of 'end-differentiated' T cells that have lost the ability to respond to specific Ag might leave PP and MLN and home to the lamina propria, where they respond to unknown stimuli. Role of dendritic cells [DC] in T and B cell responses: DCs grown from the bone marrow [BM] appear to be functionally 'immature' as compared to DCs isolated from peripheral lymphoid organs. When the latter are added to Tcell-Bcell microcultures, they augment antibody production by the B cells. However, addition of BM-DC to such cultures has no augmental effect. The main phenotypic difference between the two types of DCs are the levels of B7 and CD40, and experiments are in progress to determine which cytokines may convert the BM-DCs from an 'immature' to a 'mature' phenotype. Trans-switching to IgA in IgM-transgenic mice: Mice transgenic for a rearranged IgM heavy chain,specific for phenyl-arsonate [Ar] were immunized orally with Ar-BSA in the presence of cholera toxin, and B cells from PP cells were stimulated in vitro with T cells and DC. Clones secreting anti-Ar IgA, and bearing the idiotype of the transgene were detected, indicating that in the PP of primed mice, trans-switching to IgA can occur.